The present invention covers a printing screen and a method and apparatus for making such a printing screen. More particularly, the present invention deals with a method and apparatus for forming and precisely positioning a graphic on a printing screen.
Screen printing of graphics onto articles of clothing and other print receiving objects is a common practice. Reduced to its basics, the screen printing process comprises the steps of placing the receiving object in a printing press, mounting in the press and over the receiving object a printing screen bearing the graphic to be printed, wherein the graphic is defined by open and closed pores of the screen, and printing the graphic on the object by transferring ink through the open pores of the screen to the receiving object by means of a roller or squeegee.
In the past, it has been the typical practice to make a printing screen in the following manner. First, the graphic to be printed is formed, usually as a positive, by photographically locating the graphic on a suitable transparent sheet. An unexposed light-sensitive emulsion is applied to the printing surface of the screen (the surface of the screen in contact with the receiving object), and the transparent sheet bearing the graphic is placed over the emulsion. The regions of the emulsion not covered by the graphic carried by the transparent sheet are then hardened or "fixed" by exposure to light directed through the transparent sheet and onto the screen, thereby permanently closing the pores of the screen covered by the fixed emulsion. The unexposed regions of the emulsion are subsequently washed off of the screen to provide an open-pored area or set of areas which, in conjunction with the adjoining area or areas of closed pores, define the graphic to be printed. Once the printing screen has been prepared, the graphic is ready to be printed onto the receiving object. This is done by mounting the printing screen in the press with the printing surface of the screen resting on the receiving object and then forcing ink through the open pores of the screen.
Each time a new graphic is to be printed on receiving objects, a corresponding printing screen bearing the desired artwork must be prepared. This is a time consuming and expensive step in the screen printing process, not only because the new artwork must be formed on the transparent sheet, but also because the graphic carried by the sheet must be precisely located with respect to both the printing screen and the printing press to insure that the printed graphic is properly positioned on the receiving object.
The problem of precisely positioning the graphic with respect to the screen and the press is particularly critical when printing a multi-colored graphic on the receiving object. Such a graphic requires a number of different printing plates, one for each color in the graphic. Thus, great care must be taken to insure that the print of one color applied to the receiving object precisely registers with the prints of the other colors.
It is an object of the present invention to provide a method and apparatus for preparing a printing screen which permits any desired graphic to be quickly and easily formed on a printing screen.
It is a further object of the present invention to provide a method and apparatus wherein the graphic to be printed is automatically positioned with respect to the printing screen as the graphic is being formed on the screen.